1. Field of the Invention
This invention relates to a control device for use in a shunt motor, and more particularly to a control device for a shunt motor which facilitates speed control in a wide range from low speed to high speed a well as torque control in said range, for the purpose of minimizing the total loss in an electric motor.
2. Description of the Prior Art
Heretofore, the speed and torque control of a shunt motor, in the low speed running mode, was carried out by maintaining the field current constant and varying the armature voltage and, in the high speed running mode, by maintaining the armature voltage constant and varying the field current. However, such an attempt still suffers from the failure to achieve the control throughout the entire running range with the minimization of the total loss of an electric motor. More particularly, it has been a common practice to weaken the field only in the high speed range, while in the low range, a field current is maintained strong side, and no consideration has been given to the loss. It follows from this that in the case of controlling a wide range of speeds or R.P.M., such as for instance, in the control device of a shunt motor for use in an electric motor vehicle, there results a failure to achieve a satisfactory efficiency in the control device, with the resulting loss in mileage of a vehicle.